1. Field of the Invention
The present invention pertains to an electrically insulating sealing structure which can be used in semiconductor processing such as physical vapor deposition (sputtering). The electrically insulating sealing structure is particularly useful in processing apparatus requiring operation at a vacuum of at least 10.sup.-6 Torr and continuous use at temperatures ranging from about -10.degree. F. (-23.2.degree. C.) to about 750.degree. F. (400.degree. C.).
2. Description of the Background Art
Sputtering describes a number of physical techniques (such as DC plasma enhanced sputtering, RF plasma, and ion gun) commonly used in the semiconductor industry for the deposition of thin films of various metals such as aluminum, aluminum alloys, refractory metal silicides, gold, copper, titanium-tungsten, tungsten, molybdenum, tantalum and, less commonly, silicon dioxide and silicon onto an item (a substrate). In general, the techniques involve producing a gas plasma of ionized inert gas "particles" (atoms or molecules) by using an electrical field in an evacuated chamber. The ionized particles are then directed toward a "target" and collide with it. As a result of the collisions, free atoms or groups of ionized atoms of the target material are ejected from the surface of the target, essentially converting the target material to the free atoms or molecules. The free atoms escaping the target surface are directed toward the substrate and form (deposit) a thin film on the surface of the substrate.
A typical plasma sputtering process uses a magnetic field to concentrate the plasma ions performing the sputtering action in the region of the magnetic field so that target sputtering occurs at a higher rate and at a lower process pressure. In DC sputtering, the target itself is electrically biased with respect to the substrate on which the sputtered material is to be deposited and with respect to the sputter processing chamber. The sputtering target functions as a cathode, with the substrate (depending on the composition of the substrate), the platform on which the substrate sits, and/or the process chamber functioning as an anode. A high voltage, typically between about 200 and 800 volts, is applied between these two electrodes, with the substrate disposed upon a platform positioned opposite the cathode. The pressure in the sputter processing chamber is typically reduced to about 10.sup.-6 to 10.sup.-9 Torr, after which argon, for example, is introduced to produce an argon partial pressure ranging between about 10.sup.-2 to about 10.sup.-4 Torr. Considerable energy is used in generating the gas plasma and creating ion streams impacting on the cathode, and it is not unusual for the temperature of the internal walls, in particular the dark space shield inside the sputter processing chamber, to rise above 750.degree. F. (400 .degree. C.).
Recent developments in liquid crystal flat panel display technology have resulted in an interest in processing apparatus capable of carrying out sputtering on a particularly large scale. For example, rectangular flat panels approximately 15 in. by 19 in. (0.38 m.times.0.48 m) are not uncommon, with the industry moving toward 48 in. by 48 in. (1.2 m.times.1.2 m) panels. To achieve acceptable display performance over such a large surface area, it is necessary that the conductivity of the metallized electrodes in the underlying semiconductor device, which controls operation of the liquid crystal composition, be especially high. To achieve this high conductivity, technologists would like to use aluminum for the metallized electrodes; however, aluminum oxides form very rapidly in the presence of oxygen, contaminating the deposited aluminum layer and reducing its conductivity. To be able to sputter deposit a low stress film of pure layer of aluminum having the desired conductivity, it is necessary to carry out the sputtering operation at a particularly high vacuum. This reduces the partial pressure of residual ambient air components such as oxygen and water vapor which lead to oxidation of the depositing aluminum. For example, in a sputtering process chamber evacuated to 10.sup.-6 Torr, residual oxygen in the sputtering chamber will deposit a monolayer of oxygen upon an aluminum substrate surface within approximately one second; however, at 10.sup.-9 Torr, it takes about 1,000 seconds to deposit a monolayer of oxygen upon the substrate. This makes it desirable to deposit the conductive aluminum layer in process chambers for which the base pressure is less than 10.sup.-6 Torr, and preferably at 10.sup.-8 to 10.sup.-9 Torr, to obtain a satisfactory conductivity of a low-stress deposited aluminum layer.
FIG. 1 shows an exploded view of a sputtering process apparatus 100 of the kind used to produce flat panel display semiconductor devices. The sputtering process chamber 138 is accessed through a slit-valve opening 145 such that a substrate to be deposited upon (not shown) is delivered to a sputtering pedestal 146. The insulating structure used to insulate target assembly 124 (the cathode) from process chamber 138 (the anode) includes an outer insulator 134 and a main insulator 133. Target assembly 124 is further insulated from chamber cap 113 using upper insulator 117. Power is applied via power connection 155 which progresses through the apparatus through power connection hole 92. Vacuum passage 156 provides evacuation capability for the chamber cap 113, and a cooling manifold (not shown) included as a part of target assembly 124, provides cooling for the sputtering target. A more detailed description of this sputtering process chamber and its functioning is available in copending patent application Ser. No. 08/236,715 of Richard E. Demaray et al., filed Apr. 29, 1994, and commonly owned by the assignee of this application, which copending patent application is hereby incorporated by reference in its entirety.
Typically upper insulator 117 and outer insulator 134 are constructed from a plastic material such as, for example, acrylic or polycarbonate, as these insulators are not exposed to the high temperatures experienced by main insulator 133 and are subjected to less severe vacuum sealing requirements than main insulator 133. In the past, the main insulator 133 has been constructed from a ceramic material, for example 99.7% pure aluminum oxide (alumina) or quartz, to provide the dielectric properties required at operating conditions. Typical operating conditions expose the ceramic material to voltages as high as 1,000 volts, at temperatures which may be as high as about 750.degree. F. (400.degree. C.) and which are frequently as high as about 400.degree. F. (204.4.degree. C.). Further, main insulator 133 must also be able to sustain high compressive loads (several tons) and to make a vacuum seal with a process chamber base pressure preferably in the range of 10.sup.-8 to 10.sup.-9 Torr. Thus, the material of construction of the main insulator must meet stringent requirements.
FIG. 2A shows a cross sectional view of an assembled sputtering process chamber of the kind shown in FIG. 1. Particular detail definition in the area of main insulator 133 is shown in FIG. 2B. The target assembly 124 has a target backing plate 128 and O-ring groove 129 which is fitted with an elastomeric O-ring (not shown), typically Viton.RTM. (fluorocarbon, trademark of Du Pont Co., Wilmington, Del.), which seals against both target backing plate 128 and ceramic main insulator 133. Ceramic main insulator 133 is further sealed against sputtering chamber 138 via an O-ring groove 139 in the top flange of sputtering chamber 138, which also contains an elastomeric O-ring (not shown) constructed from a material such as Viton.RTM.. FIG. 2B shows the directional indicator P, indicating the direction perpendicular to seal which will be referred to subsequently herein.
Main insulator 133 is particularly difficult to fabricate and to handle due to the mechanical properties of the alumina from which it is fabricated. Main insulators which are 48 in. by 48 in. (1.2 m.times.1.2 m) in exterior dimension, having a width of approximately 1 in. (0.0254 m) and a thickness of about 0.5 in. (0.0127 m) are particularly difficult and costly to fabricate, and are difficult to handle without damage during sputter processing operations. Further, due to the notch sensitivity of crystalline ceramic materials, it is not practical to machine a groove into the surface of main insulator 133. Thus, the groove 129 used to provide support for the O-ring means of sealing between sputter target assembly 124 and main insulator 133 must be machined into the surface of target assembly 124. Since target assembly 124 is consumed during operation of the sputtering apparatus, the groove must be machined into each new, consumable sputtering target, adding to the cost of the target.
It would be highly desirable to have a main insulator having the necessary dielectric properties and having adequate mechanical properties to function in the application while providing ease in fabrication and handling.